48 research outputs found
Crystallization of a classical two-dimensional electron system: Positional and orientational orders
Crystallization of a classical two-dimensional one-component plasma
(electrons interacting with the Coulomb repulsion in a uniform neutralizing
positive background) is investigated with a molecular dynamics simulation. The
positional and the orientational correlation functions are calculated for the
first time. We have found an indication that the solid phase has a
quasi-long-range (power-law) positional order along with a long-range
orientational order. This indicates that, although the long-range Coulomb
interaction is outside the scope of Mermin's theorem, the absence of ordinary
crystalline order at finite temperatures applies to the electron system as
well. The `hexatic' phase, which is predicted between the liquid and the solid
phases by the Kosterlitz-Thouless-Halperin-Nelson-Young theory, is also
discussed.Comment: 3 pages, 4 figures; Corrected typos; Double columne
Energy radiation of moving cracks
The energy radiated by moving cracks in a discrete background is analyzed.
The energy flow through a given surface is expressed in terms of a generalized
Poynting vector. The velocity of the crack is determined by the radiation by
the crack tip. The radiation becomes more isotropic as the crack velocity
approaches the instability threshold.Comment: 7 pages, embedded figure
Thermodynamics of Dipolar Chain Systems
The thermodynamics of a quantum system of layers containing perpendicularly
oriented dipolar molecules is studied within an oscillator approximation for
both bosonic and fermionic species. The system is assumed to be built from
chains with one molecule in each layer. We consider the effects of the
intralayer repulsion and quantum statistical requirements in systems with more
than one chain. Specifically, we consider the case of two chains and solve the
problem analytically within the harmonic Hamiltonian approach which is accurate
for large dipole moments. The case of three chains is calculated numerically.
Our findings indicate that thermodynamic observables, such as the heat
capacity, can be used to probe the signatures of the intralayer interaction
between chains. This should be relevant for near future experiments on polar
molecules with strong dipole moments.Comment: 15 pages, 5 figures, final versio
Layers of Cold Dipolar Molecules in the Harmonic Approximation
We consider the N-body problem in a layered geometry containing cold polar
molecules with dipole moments that are polarized perpendicular to the layers. A
harmonic approximation is used to simplify the hamiltonian and bound state
properties of the two-body inter-layer dipolar potential are used to adjust
this effective interaction. To model the intra-layer repulsion of the polar
molecules, we introduce a repulsive inter-molecule potential that can be
parametrically varied. Single chains containing one molecule in each layer, as
well as multi-chain structures in many layers are discussed and their energies
and radii determined. We extract the normal modes of the various systems as
measures of their volatility and eventually of instability, and compare our
findings to the excitations in crystals. We find modes that can be classified
as either chains vibrating in phase or as layers vibrating against each other.
The former correspond to acoustic and the latter to optical phonons.
Instabilities can occur for large intra-layer repulsion and produce diverging
amplitudes of molecules in the outer layers. Lastly, we consider experimentally
relevant regimes to observe the structures.Comment: 17 pages, 20 figures, accepted versio
Análise físico-química do óleo-resina e variabilidade genética de copaíba na Floresta Nacional do Tapajós
O objetivo deste trabalho foi caracterizar o óleo-resina da copaíba (Copaifera reticulata) e estimar, por meio de marcadores microssatélites, a variabilidade genética da espécie na Floresta Nacional do Tapajós, PA. A amostragem foi realizada em duas áreas, distanciadas de 5 km, em 136 árvores. A diversidade genética foi avaliada com seis marcadores microssatélites derivados de C. langsdorffii, e o óleo obtido de 30 árvores (15 de cada área) foi caracterizado em termos físicos e químicos. O óleo C. reticulata apresenta aspecto líquido, fino, odor fraco e de coloração amarelo-dourada (73,3% das plantas), com viscosidade muito variável (18 a 187 Pa-s) e densidade média de 0,975±0,049 g cm-3. O índice de acidez variou de 9,62 a 10,17 mg g-1 de KOH e o de saponificação de 100,63 a 109,84 mg g-1. A análise molecular identificou 78 alelos, com média de 13 por loco. A heterozigosidade esperada variou 0,59 a 0,85 (média de 0,75), com nível de endogamia de 0,375 a 0,419. Houve pouca diferenciação genética entre as populações das diferentes áreas de coleta (F ST = 0,030), mas a variabilidade foi maior entre os grupos genéticos detectados pelo programa Structure (F ST = 0,070). Essa maior variabilidade indica que não há ameaças à conservação genética da copaíba, em médio prazo